Include bsd/string.h only in include/utils.h

Include bsd/string.h only in include/utils.h

This is simpler and cleaner, and avoids having to include the header
from every file where the functions are used. The prototypes of the
internal implementation are in this header, so utils.h will have to be
included anyway for those.

Fixes: 508f3c231efb ("Use libbsd for strlcpy if available")
Signed-off-by: Luca Boccassi <bluca@debian.org>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>